1. Field of the Invention
The present invention relates to the coils used in a magnetic resonance imaging system for acquiring the RF signals, and more particularly to a knee coil.
2. Description of the Prior Art
When the scan is performed using the exiting knee coils, a large number of coil units, for example, 18 coil units, are required to achieve the IPAT (Integrated Parallel Acquisition Techniques) in three directions, namely from head to foot, from left to right and from front to back (chest to back), in order to ensure the IPAT capability in the three directions.
The IPAT mentioned here refers to a technology that can save the scanning time, which has been widely adopted, including the specific examples of IPAT2 and IPAT3, wherein 2 and 3 are called IPAT factors. The work mechanism of IPAT can be simply seen as using the alternated point acquisition method to acquire signals and produce scan images. For instance, the method of acquiring a signal at every other point is called IPAT2. Due to the reduced number of signal acquisition points, the scanning time can be reduced by using the IPAT technology. Usually, to achieve IPAT2 in a given direction, the number of coil units in that direction must be equal to or greater than 2; likewise, to achieve IPAT3 in a given direction, the number of coil units in that direction must be equal to or greater than 3. Therefore, to achieve IPAT in three directions, more coil units are needed. In a normal case, each coil unit corresponds to one signal output channel, so more coil units mean more channels are required, which leads to increased cost of coils as well as work complexity.
FIG. 1 is a schematic diagram showing the structure of the existing knee coil comprising 18 coil units. As shown in FIG. 1, the knee coil as a whole is a cylindrical shape, wherein the cylinder side face is formed by a first array of coil units 101, a second array of coil units 102 and a third array of coil units 103 which are in turn adjacent to each other in the direction perpendicular to the bottom of said cylinder. In this case, each array of coil units has six coil units that are in turn adjacent to each other in the direction of the circumference of the cylinder side face (for simplicity, FIG. 1 only shows the construction of the second array of coil units 102, and the first array of coil units 101 and the third array of coil units 103 have the same construction as the second array of coil units 102). From FIG. 1 it can be seen that the knee coil is so constructed that it has IPAT3 capability in three directions. However, in practical use, each coil unit in such a knee coil needs to correspond to one signal output channel, which means that 18 signal output channels are required.
In order to resolve this problem, the existing technologies usually adopt the signal combination method. To be specific, this is to use a mixing bridge circuit to combine the signals from multiple coil units, for example, combining the signals from two coil units into one signal, or combining the signals from three coil units into two signals, and then the combined signals are output, thereby reducing the number of channels required. For the knee coil shown in FIG. 1, if there are only 8 signal output channels available, then 10 mixing bridge circuits are needed.
Although this solution can meet the need to reduce the number of channels, it has a number of problems, for example:
1) The coil has a large number of coil units (for example 18);
2) Due to the fact that the number of coil units is far greater than the number of available signal output channels, a great number (for example 10) of mixing bridge circuits are further needed;
3) Because there are many coil units and a large number of mixing bridge circuits are required, the cost of coils, the manufacturing outlay, and processing difficulty are increased greatly.